1. Field of the Invention
The present invention relates to a semiconductor light receiving element having a photodiode or a photodiode IC. In particular, the present invention relates to a semiconductor light receiving element installed in an optical pick-up device and receiving lights of a plurality of different wavelengths.
2. Description of the Related Art
An optical pick-up device is installed and used in an optical recording and reproducing apparatus such as a DVD player for playing video and audio, a DVD recorder for recording data to a DVD, and so on. Typically, the optical pick-up device is provided with a red laser diode for DVD and an infrared laser diode for CD. The red laser diode emits red light (wavelength: about 655 nm), and the infrared laser diode emits infrared light (wavelength: about 780 nm). The emitted light is irradiated on a DVD or a CD recording data, and the irradiated light is modulated and reflected by a surface portion of the DVD or the CD.
The optical pick-up device is further provided with a semiconductor light receiving element for receiving the reflected light from the DVD or the CD. FIG. 1 is a plan view schematically showing a configuration of a typical semiconductor light receiving element. As shown in FIG. 1, the semiconductor light receiving element has an infrared photodiode 1 for CD and a red photodiode 2 for DVD. The infrared photodiode receives the reflected infrared light, while the red photodiode receives the reflected red light. Moreover, as shown in FIG. 1, a plurality of infrared photodiodes 1 for CD may be provided in order to obtain a focusing function and a tracking function of the optical pick-up device in which the semiconductor light receiving element is installed. Generally, the semiconductor light receiving element is provided in the form of a photodiode IC (referred to as “PDIC”) in which the above-mentioned photodiodes and an IC are integrated on one semiconductor substrate. The IC is configured to convert a current signal into a voltage signal, amplify the voltage signal and execute an arithmetic processing.
FIG. 2 is a cross-sectional view schematically showing a structure of the typical semiconductor light receiving element. The semiconductor light receiving element (PDIC) is formed on an N-type silicon substrate 4. The infrared photodiode 1 for CD and the red photodiode 2 for DVD are formed on the N-type silicon substrate 4. Each photodiode is a photoelectric conversion element having a PN junction. Moreover, an anti-reflection film (coating) is deposited on light receiving surfaces of the infrared photodiode 1 and the red photodiode 2 such that a signal conversion efficiency of the reflected lights from the DVD and the CD is improved. The anti-reflection film consists of a silicon dioxide film 8 formed on the light receiving surfaces and a silicon nitride film 9 formed on the silicon dioxide film 8. Such the anti-reflection film with the same structure is used in common for receiving the lights of two different wavelengths. In this case, the thickness of the anti-reflection film must be designed to deviate from an optimal value for at least one of the two different wavelengths. However, in the case of the two different wavelengths of 655 nm (for DVD) and 780 nm (for CD), there exists an appropriate design value of the thickness with which the signal conversion efficiency within a practically allowable range can be achieved for both of the two different wavelengths. By forming the anti-reflection film with the appropriate design value, it has been possible to efficiently receive the reflected lights of the two different wavelengths.
In recent years, a blue laser diode emitting blue light (wavelength: about 405 nm) has come into practical use, and a larger-capacity optical disk based on blue light has been produced. An optical recording and reproducing apparatus supporting such a larger-capacity optical disk has been provided with two optical pick-up devices: one is for CD and DVD, and the other is for blue light. That is to say, the blue light has been received by a dedicated PDIC. In order to reduce the product size and the product cost, it is desirable hereafter that a single optical pick-up device is capable of processing the lights of three different wavelengths (405 nm, 655 nm and 780 nm). In other words, it is desirable that a single PDIC is capable of receiving the lights of the three different wavelengths. In this case, a sufficient signal conversion efficiency must be achieved in the one PDIC for all of the three different wavelengths. According to the conventional structure shown in FIG. 2, the thickness of the anti-reflection film must be designed to deviate from an optimal value for at least one of the three different wavelengths. It is therefore difficult to achieve high signal conversion efficiency with regard to all of the three different wavelengths.
With respect to the foregoing, related techniques are disclosed in the following patent documents.
Japanese Laid-Open Patent Application JP-P2001-307361 discloses an optical pick-up device. The optical pick-up device has: first and second light sources respective of which emit first and second laser beams from emission points of not being located on the same optical axis; an objective lens which focuses the first and second laser beams emitted from the light sources on a recording surface of an optical recording medium; and a common light receiving element which receives return lights of the first and second laser beams reflected from the recording surface. The common light receiving element has a first light receiving surface for receiving the return light of the first laser beam and a second light receiving surface for receiving the return light of the second laser beam.
Japanese Laid-Open Patent Application JP-P2001-307362 also discloses an optical pick-up device. In the optical pick-up device, laser beams of different laser wavelengths emitted from a first laser element and a second laser element are selectively supplied to a single objective lens, and signals of a first recording medium and a second recording medium of different kinds are read out. The optical pick-up device is provided with a photo-detector in which a first light receiving unit and a second light receiving unit are formed on the same semiconductor substrate. The first light receiving unit receives the laser beam emitted from the first laser element and modulated at the first recording medium. The second light receiving unit receives the laser beam emitted from the second laser element and modulated at the second recording medium. A predetermined output of the first light receiving unit and a predetermined output of the second light receiving unit can be selectively derived from the same output terminal, so that the same output terminal of the photo-detector can be used in common.
Japanese Laid-Open Patent Application JP-P2002-118281 also discloses an optical pick-up device for multi-wavelength. The optical pick-up device has: a light emitting element configured to emit a laser beam; an imaging optical system configured to focus the laser beam on an optical disk medium; a light receiving element configured to receive the laser beam and convert the received laser beam into an electric current; and an amplifier circuit configured to amplify the photo-electric current generated by the light receiving element. Specifically, the optical pick-up device has: a first light emitting element for emitting a laser beam with wavelength of 700 to 800 nm for CD; a second light emitting element for emitting a laser beam with wavelength of 600 to 700 nm for DVD; a third light emitting element for emitting a laser beam with wavelength of 500 nm or less for a high-density DVD; a first light receiving element associated with the first light emitting element; a second light receiving element associated with the second light emitting element; and a third light receiving element associated with the third light emitting element. In the optical pick-up device, any two of or all of the first, second and third light receiving elements are integrated on the same silicon substrate. With regard to at least any of the integrated light receiving elements, one electrode is formed to extend to the surface of the silicon substrate through a buried layer and a contact hole. Depths of respective buried layers are different from each other.